A field effect transistor (FET) has a gate electrode, a source electrode, and a drain electrode. It is an electronic active element for applying a voltage to the gate electrode to control a current flowing through a channel layer and switch the current flowing between the source electrode and the drain electrode.
Particularly, an FET formed by using a thin film which is formed on an insulating substrate made of, for example, ceramics, glass, or plastic, for a channel layer is called a thin film transistor (TFT).
A number of thin film transistors (TFTs) are formed on a substrate having a large area to be used in a wide variety of applications. For example, TFTs are essential elements for a flat panel display.
Conventionally, TFTs and relevant electronic devices are fabricated on a glass substrate.
Future display systems are required to be larger in size and more portable, in addition to having higher performance. As the size of the glass substrate increases, the weight of the display matters more.
One solution is to develop a display system that uses a flexible plastic substrate. In other words, a new thin film transistor technology which can fabricate a device on a plastic substrate at a process temperature lower than the current one and which provides better display performance is required to be developed.
The TFT which is most widely used at present is an element having a channel layer formed of a polycrystalline silicon film or an amorphous silicon film. For driving pixels, an amorphous silicon TFT is put into practical use, and for driving and controlling an entire image, a polycrystalline silicon TFT having a high performance is put into practical use.
However, it is difficult to form TFTs that have heretofore been developed, including the amorphous silicon TFT and the polysilicon TFT, on a substrate such as a plastic plate or film because a high-temperature process is necessary for forming a device.
On the other hand, in recent years, development for realizing a flexible display by forming TFTs on a polymer plate or film and using the TFTs as drive circuits for an LCD or an OLED has been vigorously conducted.
As a material capable of being formed on the plastic film or the like, an organic semiconductor film which can be formed at low temperature and exhibits electric conductivity has been attracting attention.
For example, as the organic semiconductor film, research and development of pentacene or the like are advanced. Its carrier mobility is reported to be about 0.5 cm2(Vs)−1, which is equivalent to the carrier mobility of amorphous Si-MOSFETs.
However, the organic semiconductor such as pentacene has low heat stability (<150° C.) and is toxic, and hence a practical device has not been realized.
Recently, as a material applicable to the channel layer of TFT, an oxide material has been attracting attention. For example, development of TFTs in which a transparent conductive oxide polycrystalline thin film mainly made of ZnO is used as the channel layer has been vigorously conducted.
The above-mentioned thin film can be formed at relatively low temperature and can be formed on a substrate such as a plastic plate or film.
However, a compound mainly made of ZnO cannot form a stable amorphous phase at room temperature, and forms a polycrystalline phase. Accordingly, it is impossible to increase the electron mobility due to scattering at polycrystalline particle interfaces. In addition, the shapes of the polycrystalline particles and the interconnections therebetween vary to a large extent depending on the film forming method, and hence characteristics of the TFT device also vary.
Recently, K. Nomura et al., Nature, vol. 432, pp 488-492 (2004-11) (hereinafter referred to as Non-Patent Document 1), reports a thin film transistor using an In—Ga—Zn—O-based amorphous oxide.
The transistor can be formed on a plastic substrate or a glass substrate at room temperature. Further, transistor characteristics of a normally-off type transistor are obtained when the field effect mobility is about 6 to 9 cm2/V s.
In addition, the transistor has a characteristic of being transparent to visible light.
In the above document, the amorphous oxide having a compositional ratio of In:Ga:Zn=1.1:1.1:0.9 (at %) is used for the channel layer of a TFT.
Reports of conventional In—Ga—Zn—O-based oxides (Non-Patent Document 1) and WO 2007/032294 A1 (hereinafter referred to as Patent Document 1)) are known as examples of a polynary oxide semiconductor that contains three different metal elements.
Those reports have attracted much attention from researchers and industry.
In order to put a TFT made from an oxide material into industrial uses, a desirable oxide material is one that allows the TFT to operate in a wide composition range (i.e., having a large composition margin) and that contains fewer different metal elements. Using an oxide material like this is very advantageous in terms of uniformity, better control of TFT characteristics, and manufacturing cost.
The drive TFT and switching TFT of an active matrix organic light emitting diode (AMOLED) are required to have high device stability and operation stability over time in addition to other required TFT characteristics.
Also requested is further to enhance the metal element composition margin (e.g., In/(In+Ga+Zn) or Zn/(Zn+In+Ga)). The present invention discloses an In—Ge—Zn—O-based oxide as an oxide semiconductor that contains three different metal elements. An effect of the oxide semiconductor (In—Ge—Zn—O-based) of the present invention when used in a TFT is that the composition margin is larger than that of a conventional oxide-based semiconductor that employs two or three different metal elements. The composition margin with respect to the TFT operation depends greatly on whether or not the channel material has electric characteristics of a semiconductor and whether or not the channel material is amorphous.
Having a large composition margin is a great advantage in mass production where a film needs to be formed over a large area and/or at high speed. Also, a semiconductor material for use in TFT is requested to be small in the amount of expensive or rare elements (In, Ga, or the like) contained therein, in order to keep the cost low.
An object of the present invention is therefore to provide a thin film transistor that solves the above-mentioned problems.